The present invention relates to a method of producing formaldehyde, particularly to a novel catalyst that permits producing formaldehyde directly from methane at a high yield and a method of producing formaldehyde by using the novel catalyst.
Formaldehyde is produced by partial oxidation reaction of methanol. Half the methanol produced in an amount of one million tons in a year is used as a raw material for the production of formaldehyde. The produced formaldehyde is used as a raw material of synthetic resins such as phenolic resins and urea resins or as a raw material of various medicines.
Methanol is synthesized from hydrogen and carbon monoxide obtained by water vapor reforming reaction of methane. The conventional process of producing formaldehyde is as given below: EQU Methane.fwdarw.H.sub.2 /CO.fwdarw.Methanol.fwdarw.Formaldehyde
The reaction for preparing H.sub.2 /CO from methane is an endothermic reaction using a large amount of high temperature water vapor, which is one of typical processes consuming a large amount of energy. On the other hand, the reaction for synthesizing methanol from H.sub.2 /CO is an exothermic reaction. In order to prevent the reaction heat from being generated excessively, the CO conversion rate must be suppressed to about 10% in operating the process unit. Also, the conversion rate of methanol must be suppressed in operating the process unit in the production of formaldehyde by partial oxidation of methanol in order to suppress formation of carbon dioxide and carbon monoxide. In short, the conventional process of producing formaldehyde is a process consuming a large amount of energy and requiring a very complex operation of the process unit.
In order to avoid the large energy-consuming process in the production of formaldehyde, it is necessary to develop a new producing process that permits producing formaldehyde without involving the step of water vapor reformation of methane to produce H.sub.2 /CO.
It is considered theoretically possible to produce methanol and formaldehyde by partial oxidation of methane, i.e., direct synthesis from methane, as suggested by chemical reaction formulas given below: EQU CH.sub.4 +1/2O.sub.2.fwdarw.CH.sub.3 OH, CH.sub.4 +O.sub.2.fwdarw.HCHO+H.sub.2 O
Therefore, vigorous researches are being made over more than these 50 years on the method of directly synthesizing methanol or formaldehyde from methane in research institutes over the world. Since a catalyst is required for the reactions given above, the major portion of these researches has been concentrated on the development of an effective catalyst. For example, catalysts having molybdenum oxide, vanadium oxide, chromium oxide, etc. supported by silica are reported in, for example, "Chemistry Letter, 1997, p31-32" and "Catalyst Today, 45, p29-33(1998)".
However, the yield of methanol or formaldehyde is very low, i.e., less than 1% in general, even in the presence of these catalysts. It is said among the researchers in this field that it is difficult for the yield of methanol or formaldehyde to exceed 4%. In other words, the yield of 4% is said to be a wall that cannot be broken in the direct synthesis of methanol or formaldehyde from methane. Further, the methane conversion rate of at least 10% is required for putting the process to practical use. In conclusion, a process for direct conversion from methane into formaldehyde with a practical yield of formaldehyde has not yet been developed.